Here is a quick example

 <Style x:Key="itemIndexSelectorStyle"
           TargetType="{x:Type xcdg:RowSelector}">
      <Setter Property="Content"
              Value="{Binding RelativeSource={RelativeSource Self}, Path=ItemIndex}"/>
    </Style>
    <Style TargetType="{x:Type xcdg:DataRow}">
      <Setter Property="xcdg:RowSelectorPane.RowSelectorStyle"
              Value="{StaticResource itemIndexSelectorStyle}" />
    </Style>

If you are refering to the RowSelectorPane itself and not its items, then you will need to redo the TableViewScrollViewer as demonstrated here: http://xceed.com/CS/files/folders/jenny/entry11522.aspx